1. Field of the Invention
The present invention relates to an electromagnetic pump.
2. Description of the Related Art
One electromagnetic pump that has been proposed includes: a piston that is inserted in a cylinder, and forms a pump chamber; an electromagnetic coil for generating an attractive force that attracts a piston; and a spring member for pressing the piston in a direction opposite to the attractive force of the electromagnetic coil by a spring force (see, e.g., Japanese Patent Application Publication No. JP-A-2007-51567). When the electromagnetic coil is not excited (OFF), this electromagnetic pump introduces oil therein by moving the piston with the spring force of the spring member. When the electromagnetic coil is excited (ON), this electromagnetic pump discharges the introduced oil by moving the piston with the attractive force of the electromagnetic coil.
Another proposed electromagnetic pump operates so that, when an electromagnetic coil is excited, the electromagnetic pump introduces oil therein by moving a piston with an attractive force of the electromagnetic coil, and when the electromagnetic coil is not excited, the electromagnetic pump discharges the introduced oil by moving the piston with a spring force of a spring member (see, e.g., Japanese Patent Application Publication No. JP-A-2009-7976).
One plunger pump that has been proposed includes: a plunger (a piston) that partitions a discharge chamber V1 (a first chamber) connected to an inlet via a check valve, and a discharge chamber V2 (a second chamber) communicating with the discharge chamber V1 via a check valve, and connected to an outlet, and reciprocates within a cylinder; an eccentric cam that moves the plunger forward by driving a motor; and a coil spring that moves the plunger backward (see, e.g., Japanese Patent Application Publication No. JP-A-2006-169993). In this pump, when the plunger is pressed by the eccentric cam to move forward, the capacity of the discharge chamber V2 decreases, and the capacity of the discharge chamber V1 increases, whereby a liquid in the discharge chamber V2 is discharged from the outlet, and a liquid is introduced into the discharge chamber V1 via the inlet. When the plunger is moved rearward by a spring force of the coil spring with rotation of the eccentric cam, the capacity of the discharge chamber V1 decreases, and the capacity of the discharge chamber V2 increases, whereby the liquid in the discharge chamber V1 is fed into the discharge chamber V2. An amount of change in capacity of the discharge chamber V1 is larger than an amount of change in capacity of the discharge chamber V2. Thus, when the plunger is pressed by the eccentric cam to move forward, the liquid is discharged from the outlet by an amount corresponding to a change in capacity of the discharge chamber V2. When the plunger is moved backward by the spring force of the coil spring, the liquid is discharged from the outlet by an amount corresponding to the difference between a decrease in capacity of the discharge chamber V1 and an increase in capacity of the discharge chamber V2.
In general, the attractive force that is obtained by electromagnetic coils is relatively weak, and the coils need to be increased in size in order to obtain sufficient pressure-feed capability as electromagnetic pumps. On the other hand, due to space limitations in vehicles for mounting a hydraulic system including an electromagnetic pump, it is desired to reduce the size of electromagnetic pumps as much as possible while improving their performance. Moreover, when the electromagnetic pumps are used continuously, generation of an electromagnetic force can become unstable due to, e.g., heat generated in an electromagnetic portion. Thus, it is desired to improve the performance of the electromagnetic pumps.